The present invention relates to a receiving apparatus and more particularly to a receiving apparatus having an AGC (Automatic Gain Control) function.
FIG. 5 illustrates a well known receiving apparatus having an AGC function. In FIG. 5, an RF (radio frequency) signal received from an antenna is inputted into an antenna tuning circuit 2 through a variable attenuator 1. The output of the tuning circuit 2 is amplified by an RF amplifier 3 and is then fed to an RF tuning circuit 4 to selectively receive a desired RF signal. The RF signal is mixed by a mixer 5 with a local frequency signal from a local oscillator 6 for frequency conversion. The output of the mixer 5 is applied to an IF (intermediate frequency) tuning circuit 7 which outputs only an IF signal. The IF signal is then supplied to an IF amplifier (not shown) through an IF narrow band filter 8.
The IF signal outputted from the IF tuning circuit 7 is supplied to a level detector 10 in a control signal generating circuit 9. The level detector 10 is so constructed that a level detection signal is outputted in accordance with the level of the IF signal outputted from the IF tuning circuit 7. The output of the level detector 10 is supplied as an AGC signal through a switch 11 to the variable attenuator 1 and the RF amplifier 3. The switch 11 is arranged, for example, to become ON in response to an ON command of a "high level." The output of a comparator 12 is supplied to the ON command input terminal of the switch 11. A reference voltage Vr is supplied to an inverting input terminal of the comparator 12. The output of a level detector 13 is supplied to a positive input terminal of the comparator 12. The level detector 13 is constructed to generate a detection signal in accordance with the level of the IF signal through the IF narrow band filter 8.
With the arrangement described above, a so-called wideband AGC is effected where an AGC signal is produced in accordance with the output level of the IF tuning circuit 7 having a wide frequency band, and the AGC signal controls the attenuation factor of the variable attenuator 1 and the gain of the RF amplifier 3. Generally, the wideband AGC can prevent intermodulation interference due to the beat components produced by a plurality of interference signals whose beat components fall within the bandwidth of the desired signal when the plurality of interference signals coexist in the RF amplifier 3. However, the signal level of the desired signal is decreased together with the interference signals due to the AGC operation to cause so-called desensitization in which the receiver becomes unable to receive the desired signal if the level of the desired signal is low. If a level of the IF signal passing through the IF narrow band filter 8, that is a level of the desired signal, is detected by the level detector 13 and the output of the level detector 13 is less than the reference voltage Vr, the comparator 12 outputs no ON command which makes the switch 11 ON. Thus, the switch 11 remains OFF causing AGC to be disabled. This prevents desensitization.
That is, in the prior art receiver, if the desired signal exists in the output of the IF tuning circuit 7, a level of a wideband output of a front end which is the output level of the IF tuning circuit increases with increasing signal level of the received RF signal as shown in a solid line in FIG. 6. When the output level of the IF narrow band filter 8 increases to a level higher than the reference voltage Vr, the switch 11 becomes ON so as to close the wideband AGC loop. At this time, if the output level of the IF tuning circuit 7 is high, the attenuation of the attenuator 1 and the gain of the RF amplifier 3 are controlled to perform the AGC operation, thereby maintaining the wideband output level of the front end substantially constant.
Conversely, if the desired signal does not exist in the output of the IF tuning circuit 7, the AGC operation is not initiated even when the received signal level increases as shown in a dash-dot line but the level of the wideband output of the front end or the interference signal increases in accordance with the level of the received signal.
As described above, in the prior art receiver, when the desired signal is low in level, AGC will not be effected even if the interference signals exist in the vicinity band of the desired signal. Thus, if the interference signals are very high in level, the prior art receiver suffers from a problem of resulting in further radio interference and noise.